Video display controllers are known which are able to work with both RGB format video and YUV format video to combine two video surfaces from graphics memory to provide an overlay output. This is useful, for example, in the situation that a movie source in YUV format is obtained by decoding a video file from say a CD-ROM or DVD device, and written to a video surface in graphics memory. At the same time a computer environment desktop is stored in RGB format in another surface within graphics memory. These two surfaces can be combined to provide the YUV source within a window on the desktop. Because the video display controller handles the conversion, scaling and mixing of the two sources, the computer operating system or application program does not directly create the desired display, but rather instructs the display controller system to scale and combine the two sources as desired. The quality of display and efficiency of the computer system is thereby greatly enhanced. An example of such a system is described in U.S. Pat. No. 5,764,201 to Ranganathan.
In such displays controller systems, only one combined output of mixed format video is possible in which case the second output can only mirror what is being displayed by the first output. Furthermore, since only two surfaces (one RGB and one YUV) are being fetched one for each of only one RGB pixel path and only one YUV pixel path, the degree of flexibility is further limited, and it would be desirable to have greater control over surface selection, combination, scaling and output.